Abstract

Recent measurements of the emission spectrum of phthalocyanine solvated in superfluid helium nanodroplets exhibit a constant splitting of each emission line relative to the absorptionspectrum. This splitting has been attributed to two distinct helium environments near the surface of the phthalocyanine molecule. Rigid-body path-integral Monte Carlo provides a means of investigating the origin of the splitting on a detailed microscopic level. Path-integral Monte Carlo simulations of -phthalocyanine at with ranging from 24 to 150 show two distinct helium configurations. One configuration is commensurate with the molecular substrate and the other is a triangular lattice. We investigate the energetics of these two configurations and use a method for calculating electronic spectral shifts for aromatic molecule-rare-gas clusters due to dispersive interactions to estimate the spectral splitting that would arise from the two helium configurations seen for . The results are in reasonable agreement with the experimentally measured splitting, supporting the existence of two distinct local helium environments near the surface of the molecule in the nanodroplets.

Received 26 April 2005Accepted 26 May 2005Published online 08 August 2005

Acknowledgments:

One of the authors (K.B.W.) acknowledges support for this research by the National Science Foundation (Grant No. CHE-0107541) and another author (Y.K.) acknowledges support by the Korea Science and Engineering Foundation through its Basic Research Program (Grant No. R01-2002-000-00326-0). Another author (H.D.W.) thanks the Air Force Office of Scientific Research and the American Society for Engineering Education for a National Defense Science and Engineering Graduate Fellowship for 2002–2005. We acknowledge R. Lehnig and A. Slenczka for ongoing valuable discussions regarding their results.